At least since the seventies processes for removing paint overspray from paint spray booths are known wherein the paint is collected in liquid streams that are continuously flowing on baffle structures. U.S. Pat. No. 3,932,151 discloses a paint spray booth for removing excess atomized paint from the air in the vicinity of a spray paint operation comprising a spray booth structure having a working or spray painting area, an exhaust chamber and an exhaust stack which together define an air passage having an inlet at the spray painting area and an outlet at the stack, within which a fan is mounted for drawing air through the inlet and out the outlet at high velocity. A plurality of generally vertical baffles are disposed in the passage between the spray painting area and the exhaust chamber for the purpose of removing overspray or excess atomized paint from the air before it is exhausted from the stack. Mineral oil is continuously flowed over the front surface of the baffles so as to entrain therein paint solids in the air stream, and the oil is received in a reservoir or holding tank wherein the paint solids are allowed to settle, and from which the oil is recirculated for downward flow over the baffles. According to the teaching of U.S. Pat. No. 3,932,151 mineral oil is essential for performing the process since the paint is not chemically modified by the oil and thus can be easily separated, and the mineral oil allows for high velocity airflow without stripping the oil from the baffles. Unexpectedly, no fire hazard or explosion hazard has been experienced with the system described in U.S. Pat. No. 3,932,151 since the paint components are incorporated into the non-flammable mineral oil.
A similar technology is known from GB-A 2 001 258 wherein also the use of oil is recommended since it is a suitable carrier that is able to completely wet the paint solids so that the paint can be recovered and reprocessed after the oil has been expressed therefrom. In the particular arrangement disclosed in GB-A 2 001 258 wherein the paint loaded gas stream is passed through a convex throat so that the gas medium and the washing liquid are accelerated with non-turbulent, unidirectional flow the washing medium may also be a mixture of water and water-miscible oil to benefit from the advantage of oil without incurring the expense attendant upon the use of oil alone. It is essential according to the teaching of GB-A-2 001 258 in order to use an aqueous system that still contains oil that rather large amounts of washing liquid are accelerated together with the gas stream in a convex throat. Thus, according to the teaching of GB-A 2 001 258 the paint loaded gas stream is contacted with a considerably large bulk volume of washing liquid that may contain water but then also has to be accelerated is together with the gas stream in a convex throat. In contrast thereto, according to the teaching of U.S. Pat. No. 3,932,151 for thin films on, for example, baffle surfaces mineral oil is essential.
In DE-A-25 51 251 a process for removing paint overspray from a spray booth is described wherein in the lower part of the spray booth two inclined plates are arranged in a V-shape type whereby both plates overlap to the extent that a slot is formed between the plates. Onto the plates a flowing film of washing liquid is applied and due to overlapping of the plates the washing liquid flows from one plate to the other creating a curtain of washing liquid which covers the gap between both plates so that the paint loaded gas stream has to flow through the curtain of washing liquid. DE-A-25 51 251 does not particularly specify the washing liquid. It is only mentioned that in case the paint contains organic solvents, like xylene and toluolene, or non-aqueous paints having aliphatic hydrocarbons as solvents are used then a denaturating agent comprising a basic compound and a polyalkylene glycol alkylmonoether is employed.
Furthermore, DE-A 43 38 003 discloses a process for the separation of organic components from air vents. According to the teaching of DE-A 43 38 003 an air vent stream comprising organic components is directed through two absorption stages wherein in the first absorption stage synthetic oils, for example, silicone oil, are used as gas washing liquid whereas in the second stage less expensive oils, like vegetable oils, can be employed. Specific examples of the synthetic oils used in the first absorption stage are silicone oil, phthalic acid dialkylesters and polyethylene glycol dialkylethers. But there is no disclosure of any kind of aqueous washing liquid.
Modern coating lines, especially in the automotive car industry, use spray booths with wet deposition of the overspray obtained. Water is sprayed to generate, within the spray booth, a finely divided water mist which takes up the paint constituents of the overspray and discharges them from the booth. The downstream water separators then collect the water loaded with paint constituents. Due to current legislation the spray booth water must be circulated. This requirement makes it necessary to incorporate treatment units for the circulating water wherein paint constituents and other contaminants are removed from the circulating water. A summary of currently used treatment systems for circulating water is given in WO 99/10284. Herein especially a method for treating circulation water from wet-wash spray booths is disclosed wherein the circulation water is passed through a reverse osmosis state. With this process a considerable reduction in energy consumption when treating the circulation water for wet-wash spray booths is achieved.
Nevertheless, due to the vast amounts of water to be circulated in an industrial coating line when using wet-wash spray booths the energy consumption of such lines and the investment costs are still very high.
WO 2008/067880 discloses a method and device for removing solids from overspray generated from painting objects. The overspray is taken up in an airstream and transported to a separating surface over which a water-free liquid with low vapor pressure at the working temperature, preferably less than 30 mbar, flows in which the solids from the overspray are taken up but not dissolved. A further requirement of the water-free liquid is that it does not react with the paint components. The paint solids are then removed from the spray booth with said liquid and separated from the liquid by sedimentation. In WO 2008/067880 it is considered essential to avoid water in the liquid for collecting the paint overspray. Suitable liquids are vegetable oils and glycol ethers.
In a press release by EISENMANN ANLAGEN BAU GMBH & CO KG, available under http://www.eisenmann.de/include/presse/Presseinformation ESCRUB.pdf, a new system for processing overspray in spray booths is disclosed. This system is a further development of the system disclosed in WO 2008/067880. According to the E-Scrub process an air stream flows substantially perpendicular to the moving direction of car bodies to be painted in down-flow mode thereby taking up the overspray that is not deposited on the car body during the spraying process. The loaded air stream is contacted below the spray booth level with inclined flood sheets that are wetted with a separating agent. The flood sheets or baffle plates are arranged in a V-shape manner without overlap. Upon contact of the paint loaded air stream with the separating agent flowing downwards the baffle plates the majority of the paint components are transferred from the air stream to the separating agent. The separating agent is collected in a pan and removed from the system. The separating agent is circulated after at least partial separation from the paint components. The air stream still loaded with paint particles is then directed to an electrostatic scrubber unit that is equipped with positively charged separating plates which are also wetted with the separating agent. High voltage wires are fitted inside the electrostatic scrubber. The paint particles in the air stream are negatively charged and attracted by the separating plates. The paint particles are taken up by the separating agent and removed with the down-flowing separating agent from the electrostatic scrubber system and are collected in the same pan as the main stream of separating agent. The press release does not contain any information with respect to the composition of the separating agent, with the exception that the separating agent promotes agglomeration of the paint particles. In the press release it is announced that the E-Scrub system allows a reduction of energy consumption in the order 78% compared to conventional wet-wash spray booth technologies.
The object of the present invention is to provide an efficient process for removing paint overspray from a paint spray booth with low energy consumption with a high recycling ratio of all carrier streams used in the process that can be run for a prolonged time with high process stability. It is furthermore an object of the present invention to provide a suitable liquid carrier for collecting paint overspray in such process for removing paint overspray from a paint spray booth.